Typically, in transmitters used in communication systems, signal noise ratio (SNR) is maintained by increasing the power of the output signal. Transmitters used in optical transmission systems include those that direct modulation laser light and those that use an external modulator. For optical transmitters that use external modulators, the optical power output by a laser diode (LD) serving as a light source, loss at the external modulator, loss through connections to passive devices such as fibers, etc. determine the power of the output signal. Further, wavelength lockers that stabilize the wavelength of light output from LDs used as light sources have been propose such as that disclosed in Japanese Laid-Open Patent Publication No. 2008-53555.
Such conventional technology, however, has a problem in that SNR maintenance for a signal is complicated because reductions in insertion loss at modulators have nearly reached their limits and because of constraints in increasing the power of light output from LDs and in reliability. For example, if schemes such as Dual
Polarization Quadrature Phase Shift Keying (DP-QPSK) are adopted, loss from multiplexing and demultiplexing LD light is great.
For example, if DP-QPSK is adopted, since LD light is branched into two to generate polarized optical signals, optical power of the signal drops by at least 3 dB. Further, compared to QPSK, the power of the output signal drops by the amount of the excess loss from the splitter that branches the output from the light source.
Whereas, providing plural LDs on the transmission side to generate polarized optical signals may stabilize the power of the output polarized optical signals, nonetheless, a problem arises in that reception of the polarized optical signals becomes complicated since respective phases and wavelengths of the light output from each of the LDs are not completely identical.
Meanwhile, for example, a homodyne receiver having optical phase locked loops (OPLLs) for each polarized beam may be considered. Nonetheless, problems of increased apparatus size and cost arise as a result of having, on the receiving side, OPLLs that respectively correspond to the LDs on the transmitting side.